Baseline concentration of microplastics in surface water and sediment of the northern branches of the Mekong River Delta, Vietnam.

The Mekong River Delta in Vietnam, is concerned by numerous microplastic-related issues such as a lack of wastewater treatment facilities and mismanagement of plastic waste released from agriculture, aquaculture and related activities. This study aimed to examine the presence of microplastics in surface water and sediment by collecting samples from six sites along the Tien River and its distributaries in February 2019. The results showed that the average concentration of microplastics over the entire area was 53.8 ± 140.7 items m-3 in surface water and 6.0 ± 2.0 items g-1 dried weight in sediment, with a predominance of microplastic fibres rather than fragments, respectively 85 % and 98 % in surface water and sediment. In the main flow of surface water, the concentration of microplastics was stable; however, in the sediment, microplastic concentration was affected by the high dynamic flow regime rather than the sources where microplastics are released.

Controlling factors of microplastic fibre settling through a water column.

Microplastic fibres are the most abundant microplastics in waterways worldwide. The settling of fibres is distinct from other particles because of their aspect ratio and shape. In this paper, we test the hypothesis that length, curliness, and settling orientation control the settling velocity of microplastic fibres in a suite of laboratory experiments. Using a Particle Tracking Velocimetry method, we measured the settling velocity of 683 polyester microplastic fibres of 1 to 4 mm in length. Experimental findings support our hypothesis that for microplastic fibre longer than 1 mm, changing settling orientation from horizontal to vertical can increase 1.7 times the settling velocity. Fibre curliness can significantly reduce the settling velocity, where a curly fibre 1.3 times longer than a straight fibre can settle 1.75 times slower. In contrast, short microplastic fibres (less than 1 mm) mostly settle horizontally, and their settling velocity is unaffected by curliness. The drag force exerting on settling microplastic fibres was analysed, and the sphere-equivalent diameter was found to be a good representation of microplastic fibre size to predict the drag coefficient. Measured settling velocity ranges between 0.1 and 0.55 mm/s and exhibits a slight increase with the increasing length of the fibres. This low-velocity range raises concerns that microplastic fibres can favour biological flocculation, form clustered aggregates with microorganisms, feed aquatic organisms and cause bioaccumulation at higher trophic levels.

Anthropogenic fibres in white clams, Meretrix lyrata, cultivated downstream a developing megacity, Ho Chi Minh City, Viet Nam.

Anthropogenic fibres are an emerging pollutant worldwide. The Can Gio mangrove area is located downstream of the Saigon River, and is characterised by high level of anthropogenic fibres originating from domestic and industrial textile and apparel manufacturing. In this area, biota is thus subjected to a high potential risk of anthropogenic fibre contamination. This study aims to characterise the accumulation of anthropogenic fibres in different tissues, i.e. gills, digestive systems, and remaining tissues, of white clams (Meretrix lyrata) cultivated in the Can Gio beach sand, during a seven-month sampling period. The results showed an average concentration of 3.6 ± 2.1 fibres individual-1 or 2.7 ± 2.4 fibres g-1 ww. Higher fibre accumulation was observed in remaining tissues than in gills and digestive systems, and no temporal variation was observed in all clam tissues. The intake of fibres by humans consuming clams was estimated to be 324 fibres inhabitant-1 yr-1.

Microplastic in atmospheric fallouts of a developing Southeast Asian megacity under tropical climate.

Microplastics in atmospheric fallouts from a developing megacity influenced by a tropical monsoon climate were investigated during a year. Three sites were selected according to the surrounding population density, the land use and occupation. The microplastic deposition fluxes varied between of 71-917 items m-2 d-1, for an observation size range of 300-5000 µm. Fibers predominated while fragments were observed occasionally. Unexpectedly, contrary to available scientific literature, deposition fluxes did not vary temporally with rainfall and wind intensity or direction, showing no effect of the tropical climate. Variations were observed between sites and were attributed to their environmental characteristics: population density and occupation space. The median length of fibers also differed between sites and could be related to in-situ fragmentation processes due to occupation space (solid waste treatment facility). Those first results from tropical climate region are showing interesting insights and are opening new perspectives on the understanding of microplastics fate from atmospheric fallouts.

Baseline assessment of microplastic concentrations in marine and freshwater environments of a developing Southeast Asian country, Viet Nam.

In aquatic environments, assessment of microplastic concentrations is increasing worldwide but environments from developing countries remain under-evaluated. Due to disparities of facilities, financial resources and human resources between countries, protocols of sampling, analysis and observations used in developed countries cannot be fully adapted in developing ones, and required specific adaptations. In Viet Nam, an adapted methodology was developed and commonly adopted by local researchers to implement a microplastic monitoring in sediments and surface waters of 21 environments (rivers, lakes, bays, beaches) of eight cities or provinces. Microplastic concentrations in surface waters varied from 0.35 to 2522 items m-3, with the lowest concentrations recorded in the bays and the highest in the rivers. Fibers dominated over fragments in most environments (from 47% to 97%). The microplastic concentrations were related to the anthropogenic pressure on the environment, pointing out the necessity in a near future to identify the local sources of microplastics.

Temporal dynamic of anthropogenic fibers in a tropical river-estuarine system.

Anthropogenic fibers, gathering synthetic fibers, artificial fibers and natural fibers are ubiquitous in the natural environment. Tremendous concentrations of anthropogenic fibers were previously measured in the tropical Saigon River (Vietnam), i.e. a river impacted by textile and apparel industries. In the present study, we want to examine the role of contrasted seasonal variation (e.g., dry and rainy seasons), via the rainfall and monthly water discharges, and of water's physico-chemical conditions on the concentrations of anthropogenic fibers in the surface water. The one year and half monthly survey evidenced that concentrations of anthropogenic fibers varied from 22 to 251 items L-1 and their variations were not related to rainfall, water discharge or abiotic factors. However, their color and length distribution varied monthly suggesting variations in sources and sinks. Based on the 2017 survey, we estimated an annual emission of anthropogenic fibers from the river to the downstream coastal zone of 115-164 × 1012 items yr-1.

Seasonality of riverine macroplastic transport.

Marine plastic pollution is an increasing environmental threat. Although it is assumed that most marine plastics are transported from land to the ocean through rivers, only limited data on riverine plastic transport exists. Recently, new methods have been introduced to characterize riverine plastics consistently through time and space. For example, combining visual counting observations and plastic debris sampling can provide order of magnitude estimations of plastic transport through a river. In this paper, we present findings from multi-season measurement campaign in the Saigon River, Vietnam. For the first time, we demonstrate that macroplastic transport exhibits strong temporal variation. The monthly averaged plastic transport changes up to a factor five within the measurement period. As it is unclear what drives the variation in plastic transport, relations between rainfall, river discharge, presence of organic material and plastic transport have been explored. Furthermore, we present new findings on the cross-sectional and vertical distribution of riverine plastic transport. With this paper we present new insights in the origin and fate of riverine plastic transport, emphasizing the severity of the emerging thread of plastic pollution on riverine ecosystems.

Macroplastic and microplastic contamination assessment of a tropical river (Saigon River, Vietnam) transversed by a developing megacity.

Both macroplastic and microplastic contamination levels were assessed for the first time in a tropical river estuary system, i.e. the Saigon River, that traverses a developing South East Asian megacity, i.e. Ho Chi Minh City, Vietnam. The analysis of floating debris collected daily on the Nhieu Loc - Thi Nghe canal by the municipal waste management service shows that the plastic mass percentage represents 11-43%, and the land-based plastic debris entering the river was estimated from 0.96 to 19.91 g inhabitant-1 d-1, namely 350 to 7270 g inhabitant-1 yr-1. Microplastics were assessed in the Saigon River and in four urban canals by sampling bulk water for anthropogenic fiber analysis and 300 µm mesh size plankton net exposition for fragment analysis. Fibers and fragments are highly concentrated in this system, respectively 172,000 to 519,000 items m-3 and 10 to 223 items m-3. They were found in various colors and shapes with smallest size and surface classes being predominant. The macroplastics and fragments were mainly made of polyethylene and polypropylene while the anthropogenic fibers were mainly made of polyester. The relation between macroplastic and microplastic concentrations, waste management, population density and water treatment are further discussed.